Ill] THE PEARL OYSTER 39 



when eyes are present in the bivalves they are 

 usually situated, not in the head region, hut 07i the 

 mantle edges, the only regions where they would be 

 of any service. Thus do necessity and use triumph 

 in nature over all else. 



The nervous system consists simply of three pairs 

 of small bodies or ganglia (which are aggregations of 

 nerve cells) connected by nerve fibres. One pair, the 

 cerebral ganglia, is situated in the head region at the 

 sides of the oesophagus. Another pair, the pedal 

 ganglia, is connected to the cerebral gangha by two 

 connectives, but the two ganglia are fused to form 

 one mass at the base of the foot. The third pair 

 is situated on the face of the adductor muscle. The 

 ganglia of this pair are connected to the cerebral 

 ganglia but not to the pedal. 



Thus the nervous system is bilaterally symmetrical 

 and composed of right and left halves. 



In this simple invertebrate nervous system we 

 find nerve fibres running out from these ganglia to 

 muscles. Other fibres, sensory in function, run in 

 from the peripheral parts of the body and the gills. 

 A stimulus such as a difference in the constitution of 

 the water, a shadow thrown on the mantle edge, or a 

 blow struck by some external object on any soft part 

 of the body, is followed by the transmission of nerve 

 impulses to ganglion cells and from these to muscles. 

 A response may or may not follow the stimulus, but 

 we can only regard it as a reflex action. 



